At present, multi-wavelength laser units, especially the multi-wavelength laser units with continuously tunable wavelengths, are extensively required in such scientific research fields as biology and materials and in such industrial fields as medical treatment and detection. The commonly seen laser units now are mostly semiconductor laser units and solid-state laser units. Due to the limitations of laser emission mechanisms and laser material performances, only a limited number of output wavelengths are available, with a narrow tunable range of usually only several nanometers, which greatly limits the application in the above fields. For example, in the process of treating tumors by using a photodynamic therapy, a photosensitizer is additionally needed to generate singlet oxygen to kill cancer cells. Since the difference between absorption peaks of laser by different photosensitizers is great, a better curative effect can only be achieved via a laser therapeutic instrument with different wavelengths or even with arbitrarily tunable wavelengths.
One of the approaches to generate laser with tunable wavelengths is to use a frequency conversion technology, for example, frequency multiplication, beat frequency, optical parametric oscillation, etc. At present, an optical parametric oscillator pumped by an all-solid-state laser unit adopts such crystals as BBO (barium boron oxide crystal), LBO (lithium baron), KTP or PPLN (periodic polarized lithium niobate) as a nonlinear frequency conversion device, such conditions as angle and position of crystals are adjusted in a mechanical manner, then laser with tunable wavelength can be generated, arbitrary wavelengths can be output based on requirements, thereby satisfying the requirements on wavelength and power in the above applications. However, since nonlinear crystals must adjust the wavelengths in a mechanically adjusted manner, leading to the fact that the device is internally provided with such parts as a stepping motor, a rotating or displacing platform, therefore, the structure is complex, the size is large, the tuning speed is low, and the stability and reliability are inferior. Another approach to generate laser with tunable wavelengths is to adopt a dye laser unit, tunable laser can be generated by utilizing the advantage that the spectral range emitted by liquid fuel is wide, the tunable range can reach up to hundreds of nanometers, the power can reach up to thousands of watts, however, the size is large and dye is harmful to human bodies and needs to be replaced periodically, therefore, such approach is rarely adopted now.